1. Field of the Invention
The present application falls within the field of production of hydrotreated liquid fuel in petroleum refining by co-processing biomass oil containing triglycerides and/or fatty acids diluted in a refinery stream of petroleum hydrocarbons for catalytic hydrotreating—HDT units. More specifically, a liquid feedstock is introduced into a first catalytic bed containing only one metal selected from group VIB and flows through at least one bimetallic HDT catalytic bed, with control of the biomass oil hydroconversion reactions, minimizing the generation of the gaseous products CO, CO2 and CH4, while maintaining the levels of hydrogenation of unsaturated hydrocarbons and of removal of the heteroatomic contaminants in comparison to the hydrotreating of the refinery stream thereof.
2. Description of the Related Art
The processing of biomass in order to obtain renewable fossil fuels is of major interest and makes a contribution towards protection of the environment.
Biomass usually yields oils rich in triglycerides, which are widely used in industry, referred to as biomass oils, including vegetable oils and animal fat lipids which provide fatty acids by various reaction mechanisms.
Natural oils and fats are comprised predominantly or substantially of triglycerides of fatty acid carbon chain having an even number of carbon atoms, saturated or unsaturated. In oils, fatty acid glycerides with unsaturated carbon chains and, in fats, saturated fatty acids glycerides predominate.
Some biomass oils are used directly as fuels or processed into biodiesel of fatty acid esters. Also, they are co-processed in petroleum refining units to produce fuels with a contribution of hydrocarbons generated from renewable sources.
Thus, the co-processing of biomass oils in petroleum refining is aimed at the conversion of triglycerides into hydrocarbons, making it possible to advantageously use an established refinery scheme with minor modifications and with a potential benefit in terms of the cost of production and distribution of bioproducts on a small scale. However, there are economic barriers in terms of the costs of production of such products in petroleum refinery co-processing, as well as technological barriers such as: the limit to the volume of biomass oil admissible in co-processing, the consumption of hydrogen for triglyceride hydroconversion reactions, and the fatty acids decarboxylation/decarbonylation reactions with the generation of undesirable gaseous products CO and CO2.
For example, the purpose of a catalytic hydrotreating unit (“HDT”) is to hydrogenate refinery streams of petroleum hydrocarbons in order to remove contaminants such as nitrogen, sulphur, oxygen and metals, and to hydrogenate unsaturated hydrocarbons. In this case the product distillation range is essentially the same as the feedstock submitted to the process, though lighter secondary products may be produced through hydrocarbon hydrocracking reactions. In particular, a conventional HDT unit aims not only to improve the quality of refinery streams but also to specify finished products such as kerosene, diesel oil and fuel oils; the processing conditions differ under the feedstock and the catalyst characteristics.
Generally speaking, a conventional HDT process comprises the flow of petroleum hydrocarbons mixed with a hydrogen stream through a fixed catalytic bed reactor, under pressure of between 1 and 15 MPa, and an average temperature of between 280° C. and 400° C. Since exothermic reactions are involved and the reactor operating under adiabatic condition it is necessary to increase the temperature throughout the fixed catalytic bed. However, the process imposes limits on the increase in temperature, normally 40° C. per catalytic bed, in order to minimize the deactivation of the catalyst and to guarantee a minimum catalyst lifetime of 1 to 2 years. When the reaction heat is very high and the increase in temperature is excessive, the reactor can have more than one fixed catalytic bed and a recycled gas stream can be injected for quenching and also for hydrogen make-up. In the case of more than one catalytic bed, the thermal release is greater in the first bed, due to the presence of more reactive components and the greater concentration of reagents, and as a result the reaction rate is higher; thus, the more refractory reagents continue reacting at a lower reaction rate through the subsequent catalytic beds in the reactor.
Therefore, a major factor in conventional petroleum refinery hydrotreating processing units, which makes the co-processing of biomass oil difficult, is the high exothermal hydroconversion reactions of the trygliceride and the generation of H2O, CO and CO2 gases due to decarboxylation/decarbonisation reactions of fatty acids.
In a fixed catalytic bed HDT reactor, bimetallic catalysts are generally charged as metal oxides (such as, Ni—Mo, Co—Mo, Ni—W and Ni—W), supported by materials with a high specific area and high porosity, with the most widely used materials being γ-alumina (γ-Al2O3) with a specific area of between 200 and 400 m2/g and a porous volume of between 0.5 and 1.0 cm3/g. Besides providing a high specific area, in which the active components are dispersed in the form of small particles, the support provides mechanical resistance and thermal stability, preventing the sintering of the catalyst within the reactor.
Such catalysts are usually sulphided in order to obtain the most activity for the catalytic bed in the process. And, as there is a synergic effect between the metal sulphides from group VIB of the Periodic Table (Mo and W) and those from group VIII of the Periodic Table (Co and Ni), the activity of a catalyst containing both is much greater than the activity of each one alone.
In summary, the co-processing of triglycerides in petroleum refinery catalytic hydrotreating units is dependent upon the characteristics of the catalysts used, the hydroconversion reactions and the molecular structure of the feedstock components.
In biomass oils catalytic processing, under hydroconversion conditions, hydrogenation of double bonds occurs, initially, followed by thermal cracking reactions of the saturated long chains of carbon atoms. In that, acrolein and carboxylic acids are generated wherein: carboxylic acids molecules can react through the decarboxylation mechanism resulting in CO2, or by decarbonylation, with the production of CO and H2O, or by the dehydration mechanism producing corresponding n-paraffins and H2O; acrolein molecules can react in the presence of the catalyst generating C3; and the CO can react with hydrogen generating CH4 and H2O.
As shown in PI 0500591-4, which is hereby incorporated by reference in its entirety, the hydroconversion of triglyceride-rich oils in a mixture with petroleum hydrocarbons, when co-processed in an HDT unit, results in an advantageous alternative which adds to the quality of the diesel oil produced. However, the generation of gaseous products (CO, CO2 and CH4) may limit refinery co-processing in fixed catalytic bed reactors charged with bimetallic catalysts, in the form of supported group VIB metal oxides, promoted by metals from group VIII, and sulphided.
A similar outcome is clear from patent document U.S. application Ser. No. 11/342,888, which is hereby incorporated by reference in its entirety, for the production of hydrocarbons with a boiling point in diesel range, such as by co-processing vegetable oils with LCO under hydrotreatment conditions in a catalytic fixed or fluid bed reactor.
Therefore, it is known that an increase in the concentration of gases (CO, CO2 and CH4) in the reaction medium decreases the catalytic activity in terms of contaminants removal and the hydrogenation of unsaturated hydrocarbons in refinery streams in order to obtain a hydrotreated liquid product. The main reason for the catalytic activity decrease in the reaction zone is due to a reduction in H2 partial pressure which takes place when there are increases in CH4 in the gas recycle.
U.S. application Ser. No. 11/962,760, which is hereby incorporated by reference in its entirety, teaches a solution for the problem of reduction in H2 partial pressure in an HDT process with the use of two reactors in series and intermediate separation of the gaseous products (CO, CO2, H2O, H2S and NH3) generated in the first reactor. This process uses a feedstock consisting of the mixture of an oil of animal or vegetable origin and petroleum hydrocarbons in order to produce a fuel containing less than 50 mg/kg of sulphur.
It is clear, therefore, that alternatives must be sought for the co-processing biomass oils with refinery streams of petroleum hydrocarbons in order to obtain finished products with a higher added value, such as: kerosene, diesel or fuel oil.
In the following, a process is described for hydrotreating biomass oils diluted in a refinery stream of petroleum hydrocarbons, offering advantages in conversion and selectivity for the production of fuels using HDT petroleum refinery units, combining the control of the triglycerides hydroconversion reactions with that of the hydrogenation reactions of the unsaturated hydrocarbons and the removal of heteroatomic contaminants from the refinery petroleum hydrocarbon stream.